The image sequence shows numbers from 1 to 50 and fades from green to red

The top decal is controlled with the "UpperSequence" Pose slider

The top decal is controlled with a SmartSkin on the "Lower Sequence" bone below the square.

Neither is controlled by time anymore.

When you make a Pose or Smartskin there is a temporary Action created in the Actions folder. That is where you can turn on "Show More than Drivers" and make entries in the "Frame" parameter of an image sequence at certain Pose slider settings or certain bone rotations.

If you had a surface feature that didn't just squish and become more obvious when bones moved but had to change shape, then that would be an occasion for using an image sequence so you could put up different forms at different bone angles.